AJAX and JSON – Lessons Learned

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4 Ιουλ 2012 (πριν από 5 χρόνια και 4 μήνες)

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AJAX and JSON –Lessons Learned
Jim Riecken, Senior Software Engineer, Blackboard
About Me
•Jim Riecken
•Senior Software Engineer
•At Blackboard for 4 years.
•Work out of the Vancouver office.
•Working a lot with User Interface components of Blackboard Learn
–Lots of JavaScript, CSS, JSP, Tag Libraries
About this presentation
–Quickly introduce AJAX and JSON.
–Describe the approaches we’ve taken for implementing asynchronous
behavior in Blackboard Learn.
–Share some experiences and lessons we’ve learned in the following
–Show “real-life” examples wherever possible from Blackboard Learn.
Quick Introduction to AJAX and JSON
Quick Introduction to AJAX and JSON
What is AJAX?•Asynchronous JavaScript And XML
–Don’t have to use XML.
–Really just an asynchronous HTTP request.
•Browser doesn’t refresh the page.

side, it’s just a normal HTTP request.

side, it’s just a normal HTTP request.
–Done via the XMLHttpRequestobject built into modern browsers.
•Various libraries are available that hide minor browser differences in how
AJAX requests are constructed.
•E.g. Prototype’s Ajax.Requestobject, Direct Web Remoting (DWR)
( “/some/url”, {
method: ‘get’,
parameters: { param1: ‘value1’, param2: ‘value2’ },
onSuccess: function( req ){
alert( req.responseText );
“Regular” RequestAJAX Request
Why use AJAX?
•Helps your application become:
–More “Desktop”-like
•Drag and drop.
•In-place editing.

More responsive

More responsive
•Instead of whole page refreshes, only parts need to update.
•Immediately show feedback that an operation is in progress and let the user
do other things while it’s processing.
–More usable
•Immediate feedback.
•No long waits.
•Less clicks.
•Better overall user experience.
What is JSON?•JavaScript Object Notation (text/x-json)
–Stems from how Object literals are specified in JavaScript.
var myObject = {
one: 'value1',
two: [ 1, 2, 3, 4, { one: 'two' } ],
three: { another: 'object' },
four: function( a ) { alert( a ); },
four: function( a ) { alert( a ); },
five: someOtherVariable
–JSON is a subset of this.
•Can have arrays and nested objects.
•No functions or variable references though.
•Property names must be strings.
{ 'property1': 'value1', 'property2': 42 }
{ 'items' : [ 'item1', 'item2', 'item3' ] }
What is JSON?
–A pretty good alternative to using XML for data transfer.
•More succinct and easy to read.
•Easy to generate.
•Don’t have to do any XML DOM processing.
•Browsers can parse it very fast into native JavaScript objects
–Using eval, or a parsing function from a JavaScript library.

E.g. Prototype’s

E.g. Prototype’s
var jsonString = “{‘
var parsed = jsonString.evalJSON( true );
alert( parsed.
) // alerts “value”
alert( parsed.
[2] ) // alerts “3”
JSON vs. XML -Format
{ '
': 'Alice', '
', 'Anderson' },
{ '
': 'Bill', '
', 'Brown' },
JSON vs. XML -Processing
XML –DOM traversal
// req is a XMLHttpRequest with
var result = req.responseXML;
var people = result.getElementsByTagName('person');
for ( var i = 0, l = people.length; i < l; i++ ){
alert( people[i].getAttribute('firstName') );
JSON –Native JS objects
// req is a XMLHttpRequest with
var result =
req.responseText.evalJSON( true );
var people = result.people;
for ( var i = 0, l = people.length; i < l; i++ ){
alert( people[i].firstName );
What are we using?
What are we using?
What are we using?
•In Blackboard Learn we use several different approaches to
asynchronous requests.
1.Direct Web Remoting (DWR)
–DWR allows Java classes to be exposed directly to JavaScript callers.
–Extremely simple.


2.Normal AJAX requests.
–Using Prototype’s Ajax.Requeston the client-side.

–Using Struts actions that return JSON content on the server-side.
–Using Json-lib to generate the JSON.–
What are we using? DWR
Service class
public class MyDwrService{
public List<String>
( String
, String
List<String> result = // construct the list somehow
return result;
In dwr.xml
<create creator=“new” javascript=“
<param name=“class” value=“my.package.MyDwrService”/>
<script type=“text/javascript” src=“/path/to/dwr/engine.js”></script>
<script type=“text/javascript” src=“/path/to/dwr/interface/
<script type=“text/javascript”>
, function( result )
alert( result[0] );
What are we using? Ajax.Request
In JavaScript
new Ajax.Request( '/path/execute/adder', {
method: 'get',
parameters: '
onSuccess: function( req ){
var result = req.
.evalJson( true );
alert( result.
Struts Action
public class AdderAction extends Action{
public ActionForward execute( ActionMapping m, ActionForm f,
HttpServletRequest req, HttpServletResponse res ) throws Exception
Map<String, String> result = new HashMap<String,String>();
int i = Integer.parseInt( req.getParameter('
int j = Integer.parseInt( req.getParameter('
result.put( '
', String.valueOf( i + j ) );
res.setContentType( "text/x-json" );
res.setCharacterEncoding( "UTF-8" );
JSON json = JSONSerializer.toJSON( result )
return null;
Lessons Learned
Lessons Learned
Be Lazy!
•Don’t do upfront processing if you don’t need to and defer
expensive processing to as late as possible.
–If a large component is hidden on the page,
•Can dynamically load the content with AJAX only when it’s accessed.
–If a JavaScript component doesn’t need to be initialized until the user
interacts with it,
interacts with it,
•Can initialize the component when the user hovers over, or puts focus on the
–If JavaScript code isn’t needed right away by the page when it loads,
•Can dynamically load scripts using AJAX.
•Example of lazy “on-demand” loading in Blackboard Learn:
–Context Menus
•Make use of the first two items in the list above.
Be Lazy! –Context Menus
•A large change in the UI of 9.0 was the introduction of
Context Menus
Be Lazy! –Context Menus
•How does it work?
–The user hovers or focuses on the context menu link.
•Event handler runs that initializes the context menu JavaScript and then
removes itself (so it only runs once.)
–The user clicks on the context menu link.

An AJAX request is sent off to the server (e.g. to a Struts action.)

An AJAX request is sent off to the server (e.g. to a Struts action.)
–Server constructs the contents of the context menu.
•Using parameters sent in the request to determine the context.
–The server returns the contents in a JSON response.
•The response is in a specific format the JavaScript knows how to process.
–The HTML elements for the menu are constructed in JavaScript
•The contents are cached.
•No need for another AJAX request if the user opens the menu again.
–The menu is shown to the user!
Be Lazy! –Context Menus
•Example loading time savings if:
–The page, without any of the context menus, takes
to generate.
–Each context menu takes
to generate in-page.
–The context menu takes
to request dynamically.
–There is a list with
context menus on it in the page.
•Latency threshold for “interactivity” is somewhere
between 50-200ms
–Anything more will feel “sluggish”
With Lazy LoadingWithout Lazy Loading
Page response time
Time to open a
context menu
Be Lazy! –Context Menus
Menu generator action
import blackboard.platform.ui.struts.dynamiccontextmenu.BaseContextMenuGenerator;
public class MyGenerator extends BaseContextMenuGenerator
public class MyGenerator extends BaseContextMenuGenerator
protected List<List<ContextMenuItem>> generateContextMenu( HttpServletRequest request )
List<List<ContextMenuItem>> result = new ArrayList<List<ContextMenuItem>>();
List<ContextMenuItem> group = new ArrayList<ContextMenuItem>();
result.add( group );
String param1 =
ContextMenuItem item = new ContextMenuItem();
group.add( item )
return result;
Be Efficient!
•Try to minimize the number of HTTP requests that you need to
do while loading the page.
–Many small requests are slower than one big one.
•Combine related JavaScript functions into one file.
•Don’t add an external script file more than once.

Can even load JavaScript code on demand.

Can even load JavaScript code on demand.
•Get code with an AJAX request, then dynamically construct a <script>
block and add it to the <head>.
•Ensure that your scripts and content (if desired) can be cached.
–Add appropriate Cache-Controlor Expiresheaders.
–With AJAX requests
•Use GETrequests for content that may be cached –browsers will cache
AJAX GETs unless told otherwise.
•Use POSTrequests for saving.
Be Efficient!
•Minimize time spent in “onload” initialization JavaScript code.
–Some browsers execute JavaScript muchslower than others.
–Long-running scripts can cause the page to appear unresponsive.
–Ways to help:
•Split up processing into chunks, waiting a short amount of time between
each chunk to let the browser do other things.
each chunk to let the browser do other things.
–Use setTimeout, or Prototype’s Function.delay orFunction.defer
•Perform lazy initialization of components
–Like the context menus do.
document.observe("dom:loaded", function(){
(function() {
//Do first part of long-running item.
(function() {
//Do second part of long-running item.
delay( 0.1 );
delay( 0.1 );
Be Efficient!
•Batch calls together if you’re using DWR.
–Instead of N small AJAX requests, it will do one large one that
performs all the operations in the batch.
MyService.method1( ....., callback1 );
MyService.method2( ....., callback2 );
–Instead of 12separate AJAX requests, this only makes 1
•All the callbacks will be called with the method call’s
return value when the overall request completes.
MyService.method2( ....., callback2 );
for ( int i = 0; i < 10; i++ ){
MyService.method3( i, callback3 );
•If data is not likely to change often, cache it. E.g.
•Results of database queries.
•Generated JSON.

Results of AJAX calls

Results of AJAX calls
–Can do this either with browser caching (by setting appropriate Cache-Control
–Or saving data in JavaScript objects.
•Disk space (and increasingly, memory) is cheap.
•Time is expensive.
•Example of JSON caching in Blackboard Learn:
–Blackboard Grade Center
Cache! –Grade Center
•The Blackboard Learn Grade Center makes heavy use of AJAX and
–Grade data is pushed to the browser and then saved to the server
–Updates to grade data are retrieved incrementally from the server.

Generating the JSON data needed to render the Grade Center is

Generating the JSON data needed to render the Grade Center is
very expensive, especially for large courses.
columns =
pieces of grade data.
–Database queries, calculations, etc.
•The grade data is cached at two levels:
–On the browser–Until the instructor leaves the current course.
•Helps with secondary loads of the Grade Center.
–On the server–JSON in files in the file system.
•Helps with initial load.
Cache! –Grade Center
•Client-side caching
–Data is stored in JavaScript variables inside the frameset.
–Not 100% reliable.
•Doesn’t work with selective SSL due to browser security handling.
•Doesn’t work if the page is opened outside the frameset.

But it helps a lot in most cases.

But it helps a lot in most cases.
Cache! –Grade Center
•Server-side JSON caching.
–Although the client side caching helps, it doesn’t help with the loading of
the grade data when entering the grade center initially after login.
•Database queries.
•Generation of JSON.

Gradebook data doesn’t change that often

Gradebook data doesn’t change that often
•So, the JSON can be cached on the file system.
•Then sent back to the browser on subsequent entries.
•Along with a “delta” of what’s changed between the cached version and the
actual live data in the database.
–The caching gives a significant performance improvement
Cache! –Grade Center
•How the caching works
–When the cached JSON data doesn’texist:
•It is generated (e.g. all the database queries are run)
•It is sent back to the browser while simultaneouslybeing streamed to the file
•The browser shows the grade center grid.
–When the cached JSON data doesexist:
•It is sent back directly to the browser and a delta computation is immediately
started (via a background thread.)
•The browser retrieves the data and then requests the delta from the server.
–In most cases, the delta has finished processing by the time the request is made.
–If not, the request will wait until the delta is complete.
•The delta is sent back to the browser.
•The browser shows the grade center grid.
Cache! –Grade Center •Performance improvement when
data is cached:
–Small Course
•40 students
•26 columns
•No caching: 1828ms

Caching: 1421ms

Caching: 1421ms
•Improvement: 23%
–Extreme Course
•500 students
•98 columns
•No caching: 44827ms
•Caching: 16594
•Improvement: 63%
No Cache
Trust No One
•AJAX requests and responses are just like any other HTTP
–Don’t assume input from an AJAX request is clean.
•Assume the opposite.
•Filter all input that may be displayed somewhere later.

Validate request parameters.

Validate request parameters.
•Client-side validation can be trivially bypassed.
–Check that a user is allowed to do the action that they are trying to do
•If they figure out the URL, students can try to call an instructor-only action.
•Ensure that the user has the right entitlements.
Stop Cross Site Scripting (XSS)
•Cross-site scripting happens when arbitrary JavaScript code is
allowed to be injected into parts of a page.
–Attacks can perform actions as other users
•If the instructor looks at an “infected” page, for example, the script could try
to make all the student users instructors without the instructor knowing.

Ways to avoid:

Ways to avoid:
–Strip out HTML from alluser input
–If you need allow HTML, run XssUtil.filter( String input )on
allHTML inputs.
•It strips out all known XSS attacks from the HTML while still allowing most
tags through.
Stop Cross-Site Request Forgery
•Cross-Site Request Forgeries (XSRF) are a subset of XSS
attacks where the attack originates on another website
–Instructor visits “Site X” which has an XSS attack on it that makes a
request to the Blackboard Learn server to make all the students
–Since the instructor is logged in to Blackboard Learn, the request
•Ways to fix
–Use POSTs for actions that modify data
•Marginally more difficult to attack.
–Include a unique token (or “nonce”) generated each time a request is
made (that the server knows)
•The server checks for the existence of this parameter,
and rejects requests that don’t have it.
XSRF –Cont.
–For AJAX requests, you can “double submit” cookies.
•Cookies are automatically sent in request headers.
•JavaScript can access cookies, but only for the domain that the current
page is on.
•In the AJAX request, send the document.cookieas a request parameter
and on the server side check if it matches what’s in the request header.

DWR > 2.0 does this by default on all requests.

DWR > 2.0 does this by default on all requests.
new Ajax.Request('/url/to/action',{
method: 'post',
parameters: {
: document.cookie, .... },
onSuccess: function() { ... }
//in action code
String cookie = request.getHeader("Cookie");
String jsCookie = request.getParameter("
if ( StringUtil.isEmpty( jsCookie ) || !jsCookie.equals( cookie ) )
Design for Accessibility
•Not all users have sight, or can use a mouse.
•Ensure your rich UI components are keyboard accessible
–E.g. Drag and Drop in Blackboard Learn
•Completely separate set of controls that allow reordering of items to be
performed using the keyboard.
Design For Accessibility
•Ensure components are described appropriately.
–Give form elements <label>s
–Can give additional context to a screen reader by placing content off-
screen with CSS
<span style="
position: absolute; top: -10000px; left: -10000px;
">Info for screen

Give feedback that operations have completed.

Give feedback that operations have completed.
–Show an alert, or set focus on a message.
–E.g. Inline Receipts in Blackboard Learn
•Are focused on when they appear.
•ARIA (Accessible Rich Internet Applications) is an emerging
W3C standard for accessible “Web 2.0” sites.
–Specialized HTML markup lets assistive technologies (e.g. screen
readers) know what role a given component on the page has.
•E.g. that a specific <ul>is actually a popup menu

Live regions in specific for AJAX.

Live regions in specific for AJAX.
–They indicate areas of the page that will be updated with new content
(e.g. from an AJAX call)
–They will notify the user when the content has updated based on the
mode the live region is set
•“polite” –announce updates when the user is idle.
•“assertive” –announce updates as soon as possible.
Q & A